Well Terd has had alot better luck in his tank than I have.
I found that in my 90g all I could have was 1 jag, a convict and a couple bristlenose plecos and this only worked becouse I had plenty of hiding spots for during tantrums. The female jag and the male convict had seemingly paired off but she still on occasion tried to kill him.
In my 180g I started off with 2 JDs, a male festae, an amphilophus lyonsi, Pacu, Firemouth, GT, FH, Leporinus fasciatus, 3 pictus cats, 15 cory cats, and a sailfin pleco. The FH was the first to go at 6" when he turned homocidal and tore up everyones fins and scales (no casualties luckily). Then everything was fine until my smaller JD went through a major growth spurt from 7.5" to 9.5" in 2 months at which point he decided the tank wasnt big enough fort the other 8.5" JD so he was removed after about a week of liplocking that started out small but ended up very brutal. The Pacu is 14" and can take a beating which is good because he gets beaten by everyone. Pictis cats killed several cory cats so the survivors got transfered into the peacock tank and were replaced by a couple lobsters (excellent armored cleaning crew as long as they have plenty of good hideouts to moult in).
So for now the 180 is "peaceful" again with a 9.5" JD, a 11.5" RT, 6" GT, 4.5" Leporinus fasciatus (constantly at it with the firemouth), 4.5" Firemouth, 10" Lyonsi, 14" pacu (still wishes he was dead), 14" sailfin pleco, 3 5" Pictus cats (Despite the recommendation to keep in groups of 3 they are vicious to each other), and 2 5" lobsters.
IMO the best setup is 1 large agressive cichlid, I small vicious cichlid (convict or firemouth) LOTS of hiding spots the large cichlid cant get into, a large pleco 10"+, and a lobster (these guys clean a tank amazingly, even the fish feces is shredded and instead of settling on the bottom it goes out to the filter! They are a must have, they make cleaning easy).
I realy wish I had better luck overstocked but I didnt. More power to those of you who can make it work.